Conventional ophthalmic diagnostic techniques are based on recording bioelectric responses generated by the retina, the optical nerve and all cellular and nerve processes including the central visual cortex, in association with a visual stimulus perceived by the patient. These bioelectric responses are recorded, through suitable electrodes arranged at the level of the conjunctival fornix, cornea, or, in the case of binocular recording, surface electrodes near the central visual cortex of both eyes, from which a biopotential can be obtained. The responses represen a measurement of the integrity of the visual system (density of cones, rods and cells connected thereto, gangliocytes, retinal cells, nerve fibres and visual cortex), or of possible alterations or destructive actions already caused by various pathologies.
Using current techniques (such as those set forth in: (i) U.S. Pat. No. 4,846,567 which issued on Jul. 11, 1989 to Sutter; (ii) E. B. Brown et al., Contrast luminance as parameter defining. . . . etc. Ophthalmic and physiological optic, Vol. 16, n. 1, January 1996, pp.42-48; and/or (iii) E. E. Sutter et al.: “The Field Topography of ERG Components in Man”, Vision Research, Bd. 32, Nr. 3, 1992, Seiten 433-446) it is considered possible to record a number n of biopotentials (i.e., responses generated at different locations not just those from the retina) suitable for mapping the topography of the retina or central visual cortex.
Although widely used in current ophthalmologic diagnosis, this technique, it has been found, has limitations, including a lack of control of the retinal and/or consequently cortical areas actually stimulated, and the instant feedback of the resulting bioelectric responses recorded. In an attempt to stimulate and, thereby, record the biopotential generated by the ocular areas of interest, methodologies have been developed such as that set forth in E. E. Sutter et al., “The Field Topography of ERG Components in Man”, Vision Research, Bd. 32, Nr. 3, 1992, Seiten 433-446. E. E. Sutter et al.s' method is based on stimulation through “m-sequences” and the subsequent decoding through cross-correlation between the stimulated area and the resulting biopotential (retinal or cortical reaction) recorded. While this technique has been found useful, it not only requires long and extended execution times, but also, in allowing possible artifacts (e.g., biopotentials induced by loss of focus or movements of the patient's eyes) to be monitored, does not allow for their rapid correction. This can the, in turn, adversely affect subsequent results, as is widely documented in the Guidelines for Basic Multifocal Electroretinography (mfERG) Documenta Ophthalmologica, n. 106, pp.105-115, 2003.
Other improvements in conventional techniques include fragmentation of the m-sequence into steps of shorter duration that allow an intermediate evaluation point for monitoring the accuracy of the result. A method of this general description is disclosed, for example, in U.S. Pat. No. 6,022,107, which issued on Feb. 8, 2000 to Ernst Kutschbach et. al. While this approach enables sampling, it has been found unreliable due to a discarding of automatism of the bioelectric responses exceeding predetermined size limits—to be repeated. This technique, in addition, requires a delay to the end of each m-sequence cycle for some repetitions to occur, thereby imposing long and unnecessary waiting times on the patient.